1. Field of the Invention
This invention relates to a sensor voltage reading circuit which is coupled to an analog sensor, and more particularly to a sensor voltage reading circuit which removes a noise voltage, caused by electric noise, superimposed at an input terminal of an analog/digital conversion circuit of the sensor voltage reading circuit. This invention also relates to a sensor voltage reading circuit wherein a voltage is applied to a power source terminal of a sensor only when the output voltage of the sensor is to be read to suppress the power dissipation by the sensor.
2. Description of Background Art
Conventionally, in a field wherein a physical amount is detected as an electric amount, a method of detecting the physical amount by means of an analog sensor is commonly employed which includes converting the output of the analog sensor into a digital value by means of an analog/digital (hereinafter referred to as A/D) conversion circuit and processing the digital value by calculation processing by means of a central processing unit (hereinafter referred to as CPU).
In this instance, a noise removal circuit is interposed between the analog sensor and the A/D conversion circuit to remove a noise voltage superimposed on the output voltage of the analog sensor.
However, a power source terminal of the analog sensor is in most cases connected in parallel with a power source terminal of a controller which is formed by the A/D conversion circuit and the CPU. Consequently, if a voltage is applied to the power source terminal of the controller, then the voltage is also applied to the power source terminal of the analog sensor. While it is possible to remove a noise voltage superimposed on the output voltage of the analog sensor, there is a disadvantage in that electromagnetic wave noise inputted from the input terminal of the A/D conversion circuit cannot be removed.
Furthermore, since a voltage is applied simultaneously to the power source terminal of the controller and the power source terminal of the analog sensor, and also since a voltage is applied even when the CPU does not read the output of the analog sensor, a voltage is always applied to the power source terminal of the analog sensor. Therefore, there is a problem in that power is dissipated by the analog sensor.